You still have not proved that the rate of decay has been constant through time. Sure--it's been constant for the last couple of hundred years of so. But that's all you can prove. You cannot prove that it's been at this same rate through thousands (much less millions) of years.
You did not understand what I posted concerning radiocarbon dating.
The tree ring calibration would show any changes in the decay rate--if there was one. There is not. Again, the only folks proposing such changes are creationists who disagree with the results of radiometric dating and so are grasping at straws to find any flaws in the various methods.
The tree ring calibration does show some fluctuations in the atmospheric constant, and allows correction for those. That is why you end up with a calibration curve rather than a straight line.
Radiocarbon dating is useful back some 50,000 years. Other forms of radiometric dating must be used for older dates. That is outside my area of expertise.
But we can show that there's no evidence for accelerated decay. The faster isotopes decay, the more heat and radiation they give off. In order for the dates to support a young earth, the decay would have to have been accelerated to the point that the heat and radiation given off would have (quite literally) cooked everything on the surface of the earth. No plants, no animals, no oceans - nothing. Nothing but a big molten ball of radioactive rocks, anyway. However, here we are, not having been cooked off, and we have evidence for life in antiquity, ergo radioactive decay cannot have been accelerated to the point required to support young-earth creationism.
Of course, this does not prove the constancy of decay rates, but it does blow a pretty big hole in the notion that they could have been accelerated to the point that it materially affected our understanding of the age of the earth. Accelerated decay has consequences, so the easiest way to check it out is to look for evidence of those consequences. The effects of accelerated decay we would expect to observe do not exist, ergo, the evidence weighs against it.